1. Technical Field
The present invention relates to a filter, more particularly to a tunable filter that can vary its filter properties, such as the center frequency and bandwidth of the filter, by utilizing a sliding system.
2. Description of the Related Art
A filter is a device for passing signals of only a certain frequency band from among the inputted frequency signals, and is implemented in various ways. The band-pass frequency of an RF filter may be determined by the inductance and capacitance components of the filter, and the operation of adjusting the band-pass frequency of a filter is referred to as tuning.
In a communication system, such as a mobile communication system, certain frequency bands may be allotted to certain businesses, which may divide the allotted frequency bands into several channels for use. In the related art, communication businesses generally manufacture and use a separate filter that is for suitable for each frequency band.
In recent times, however, rapid changes in the communication environment have created a need for a filter to have variable properties, such as for the center frequency and bandwidth, for example, unlike the earlier environment for mounting filters. For varying the properties in this manner, a tunable filter may be used.
FIG. 1 illustrates the structure of a tunable filter according to the related art.
Referring to FIG. 1, a filter according to the related art may include a housing 100, an input connector 102, an output connector 104, a cover 106, and multiple numbers of cavities 108 and resonators 110.
An RF filter is a device for passing signals of only a certain frequency band from among the inputted frequency signals, and is implemented in various ways.
A number of walls may be formed within the filter, with the walls defining cavities 108 in which to hold the resonators, respectively. The cover 106 may include tuning bolts 112, as well as coupling holes for coupling the housing 100 with the cover 106.
The tuning bolts 112 may be coupled to the cover 106 and may penetrate inside the housing. The tuning bolts 112 may be arranged on the cover 106 in corresponding positions in relation to the resonators or in relation to particular positions inside the cavities.
RF signals may be inputted by way of the input connector 102 and outputted by way of the output connector 104, where the RF signals may progress through the coupling windows formed in the cavities, respectively. Each of the cavities 108 and resonators 110 may generate a resonance effect of the RF signals, and this resonance effect may filter the RF signals.
In a filter according to the related art, such as that shown in FIG. 1, the tuning of frequency and bandwidth may be achieved using the tuning bolts.
FIG. 2 is a cross-sectional view of a cavity in a tunable filter according to the related art.
Referring to FIG. 2, a tuning bolt 112 may penetrate through the cover 106 to be located above a resonator. The tuning bolt 112 may be made of a metallic material and may be secured to the cover by way of screw-coupling.
Hence, the tuning bolt 112 can be rotated to adjust its distance to the resonator, and by thus varying the distance between the resonator 110 and the tuning bolt 112, tuning may be achieved. The tuning bolt 112 can be rotated manually, or a separate machine for rotating the tuning bolt 112 can be employed. If the tuning achieved at an appropriate position, the tuning bolt may be secured using a nut.
FIG. 3 is a diagram illustrating the principle on which tuning is achieved by rotating a tuning bolt.
Referring to FIG. 3, capacitance (C) is formed between the tuning bolt and the resonator. Capacitance is a physical property that varies depending on the permittivity between two metals, the cross-sectional areas, and the distance. Here, the distance corresponds to the distance between the tuning bolt and the resonator.
Thus, as the distance between the tuning bolt and the resonator is changed due to the rotation of the tuning bolt, the capacitance can also be changed. Capacitance is one of the parameters that determines the frequency of a filter, and therefore the center frequency of a filter can be changed by altering the capacitance.
Although this filter according to the related art is structured to enable tuning by utilizing tuning bolts, it is considerably difficult for a user to tune the filter's properties using tuning bolts. In practice, tuning a filter using tuning bolts was usually performed only by the filter manufacturer to fine-tune the properties after the manufacture of the filter, as it was difficult for the user to perform the tuning.